Conventionally, it is known to use a drilling cement slurry for collecting natural resource deposits such as petroleum or natural gas. The drilling cement slurry is used for filling a space (annulus) between a casing pipe for well drilling and a wellbore and fixing the casing pipe. The cement slurry is injected through the casing pipe, enters the annulus from the well bottom, and hardens. The well inner wall is protected by this step called cementing. As the cement slurry, those having a low viscosity and capable of providing easy filling are suited.
However, according to this method, the cement slurry comes into contact with the well wall surface, leaving a fluid loss problem that water in the cement slurry flows out into a porous stratum or rock. When water in the cement slurry is lost due to fluid loss, the slurry viscosity rises to reduce the fluidity, giving rise to plugging of the annulus. And the flow of water into stratum causes collapsing stratum. Furthermore, the water/cement ratio in the cement slurry is changed, leading to a cement hardening failure.
In order to solve this problem, it is known to use a polyvinyl alcohol-based resin as a dehydration-preventing agent for reducing the fluid loss. However, this technique is not sufficient in that the resin is dissolved at a high temperature to cause performance reduction of the dehydration-preventing agent. On the other hand, a method of using a polyvinyl alcohol-based resin of which non-dissolvability is increased has been proposed.
For example, Patent Document 1 has proposed a method of using a polyvinyl alcohol-based resin and a sulfonic acid-based resin in combination. In addition, Patent Document 2 has proposed a method of using a modified polyvinyl alcohol-based resin crosslinked by a specific structure. Furthermore, Patent Document 3 has proposed a method of using a polyvinyl alcohol-based resin having a high saponification degree, a high polymerization degree, and a low 1,2-glycol bond content, which may contain an ethylene structural unit.